We performed statistical analysis using IBM SPSS version 19.0. Most of the metal concentrations in the air samples were highly skewed, and therefore, we log (ln) transformed them and used parametric statistics to evaluate the results. We analyzed all measurements from occasions 1 and 2 together. For correlation analysis between concentrations in air samples and exposure biomarkers, we used the inhalable fraction because it best describes the fraction of particles that the workers actually inhale during breathing. We used non-parametric statistics on nontransformed data for the biomarkers